The term “satellite system(s)” referred to hereinbelow, should be understood to encompass any one or more members of the group that consists of geo-stationary satellite systems, Low Earth Orbit (“LEO”) satellite systems and Medium Earth Orbit (“MEO”) satellite systems and other types of platforms such as High-Altitude Platforms (“HAP”) which are quasi-stationary aircrafts that provide means of delivering a service to a large area while remaining in the air for long periods of time, High-altitude, long-endurance unmanned aerial vehicles (“HALE UAV”), and the like.
In a typical satellite communications network a portion of the available capacity is allocated for hub-to-satellite communications in the forward link. Similarly, a portion of the return link capacity is allocated for satellite-to-hub communications. Although these portions of the link capacity, allocated for communicating with the hub, (also referred to as an earth station, gateway or teleport), are not discussed explicitly in the following description, still, it should be noted that the methods and air interface protocols discussed in the following disclosure may as well, and typically are, implemented in such a hub, in case where the satellite serves merely as a “bent pipe”. Namely, the satellite does not process the signals it receives other than carrying out a basic filtering thereon and shifting them in frequency.
In various satellite communication systems Frequency Division Duplexing (FDD) is used. Different frequencies are used for the forward traffic (i.e. traffic transmitted from the satellite to the terminals) and for the return traffic (i.e. traffic transmitted from terminals to the satellite) of the RF link.
Typically, for the capacity portions allocated in the uplink (namely, for transmitting hub to satellite communications and terminal to satellite communications) the allocated frequency is substantially different from the frequency allocated for carrying out downlink communications (i.e. satellite to hub communications and satellite to terminal communications), using the capacity portion allocated therefor.
General Description
In various communication systems/network terminals that cannot receive traffic while they are transmitting traffic. In order to accommodate this limitation, and at the same time make efficient use of both uplink and downlink capacity, the system/network must perform specialized forward link multiplexing and return link capacity assignment.
The term terminal refers in general to an end station of a communication system connected to the end user of the system. In the context of a two way satellite communication system the term refers to the ground station used by the consumer while the term hub or gateway refers to the ground station which serves the service provider.
One approach for scheduling the transmissions is to perform on-the-fly transmit-receive conflict resolution without imposing any limitation on the terminals by inducing a framing mechanism thereon. To do that, a scheduler must ensure that packets are only multiplexed onto the forward link at such times that they arrive at the terminal when it is not transmitting. This means, in turn, that the forward link multiplexer must maintain a separate queue for each (active) terminal and, in addition, track the propagation delay between the satellite and that very same terminal. Once every return link time slot, and for each non-empty output queue, the scheduler would use the delay information to consult the return link capacity allocation matrix in order to check whether, at the projected time of forward link packet reception, the terminal is scheduled to transmit or not. The scheduler must then serve fairly the non-blocked queues. In addition, scheduling must allow terminals certain pre-agreed short transmission windows for random-access return link transmissions. Finally, return link capacity allocation must keep a terminal's transmission duty cycle below 100% to ensure that it can send forward link traffic without excessive delay.
Transmit-receive scheduling also impacts terminal handover between beams and satellites. With the scheme described above, the scheduler must be involved in each handover in order to make sure that forward link data is correctly re-routed.
It is an object of the present disclosure to provide a transmit-receive framing mechanism that simplifies substantially scheduling, streamline satellite and beam switchover.
It is another object of the present disclosure to provide a transmit-receive framing mechanism in which most of the complexity involved in routing and handover is shifted from the satellite to the gateway and the terminals.
It is still another object of the present disclosure to provide a novel method for enabling communications between one or more satellites and a plurality of terminals, wherein the plurality of terminals are divided into M groups of terminals.
According to a broad aspect of the present invention there is provides communication terminal adapted for receiving a plurality of designated communication sub-frames transmitted in a forward link from a satellite and/or from a data gateway and/or from another data communication mediator, to the terminal. The communication terminal is associated with a certain group of one or more respective groups of communication terminals, and each designated communication sub-frame is a respective portion of a communication frame, which transmitted from the data communication mediator (e.g. satellite) in the forwards link. The designated communication sub-frame includes N communication sub-frames designated to serves respective one or more groups of communication terminals. The satellite communication terminal includes:                (i) a scheduling module configured and operable for determining a time slot of the designated communication sub-frame within the communication frame transmitted by the communication mediator (satellite). The scheduling module may for example include:                    a forward link scheduler configured and operable for assigning a forwards link schedule for receiving said designated communication sub-frame at said time slot; and            a return link scheduler configured and operable for assigning a return link schedule for transmitting information to the satellite during time slots other than said time slot of the designated communication sub-frame; and                        (ii) a signal receiving module associated with the scheduling module and configured and operable for performing signal receipt operation during the forwards link schedule for receiving and processing said designated sub-frame of the communication frame transmitted in the forward link from the communication mediator (satellite).        
According to some embodiments the signal receiving module includes a signal acquisition system configured and operable to process at least a part of the communication frame received in the forward link from the communication mediator (satellite) and to lock on to the designated communication sub-frame by identifying at least one code word in the received signal designating the designated sub-frame, and determining a time index (sample position) at which the code word is encoded in the received signal and a carrier frequency over which the code word is encoded in the received signal.
According to another broad aspect of the present invention there is provided a signal acquisition system. The signal acquisition system includes:    (i) an input module adapted to obtain a received signal (e.g. EM signal), which encodes communicated data over a certain unknown carrier frequency. The certain unknown carrier frequency may be any one of a plurality of possible carrier frequencies residing within a predetermined frequency band.    (ii) a signal time frame processor connectable to the input module and configured and operable for continuous processing of time frame portions of the received signal to identify at least one code word of a group of one or more predetermined code words, being encoded in a time frame portion of the received signal. The signal time frame processor includes:            a. a carrier frequency analyzer module configured and operable for analyzing the time frame portion of the received signal in conjunction with the plurality of possible carrier frequencies simultaneously. This is achieved by transforming the time frame portion to generate carrier-data including a plurality of carrier-data-pieces associated with each possible carrier frequency of the plurality of possible carrier frequencies respectively. Each of the carrier-data pieces is indicative of data encoded in the time frame portion over a carrier frequency associated with the respective carrier-data piece; and        b. a convolution module configured and operable for processing the time frame portion of the signal to simultaneously identify whether the time frame portion encodes said at least one code word, over any one of the a plurality of possible carrier frequencies.        
The signal acquisition system also includes an output module configured and operable for outputting identification data indicative of identification of said code word in the signal.
To this end, the signal acquisition system is adapted to determine a time index of said code word in the received signal based on the time frame portion of the received signal at which said the code word is identified, and the output module is adapted to output the time index. The time frame processor is adapted to process the carrier data to identify the carrier-data piece, which encodes significant data and thereby determine the carrier frequency of the received signal. The output module is further adapted to output the determined carrier frequency.
The invention also provides a satellite communication terminal adapted for receiving a plurality of designated communication frames transmitted in a forward link from a satellite to said terminal, wherein said satellite operates in a beam-hopping mode and said communication terminal is associated with a certain group of one or more respective groups of communication terminals associated with respective beams transmitted by said satellite in said beam-hopping mode;                wherein the satellite communication terminal comprises a signal receiving module configured and operable for performing signal receipt operation during a forwards link transmission of a respective beam of the beam-hoping mode which is associated with the certain group for receiving and processing the communication frame transmitted in said forward link from said satellite; and        wherein said signal receiving module comprises the above-described signal acquisition system configured and operable to process at least a part of the communication frame received in the forward link from said satellite and to apply carrier locking on to a carrier frequency of said respective beam by identifying at least one code word in the respective communication frame and determine a time index at which said code word is encoded in the received signal and a carrier frequency over which said code word is encoded in the received signal.        
Other objects of the invention will become apparent as the description of the invention proceeds.
In the following description it is assumed that the air interface's forward link uses one or more TDM carriers, whereas its return link uses a reservation access scheme such as Multi-Frequency Time Division Multiple Access (MF-TDMA).
A key aspect of the air-interface of the present disclosure is its ability to accommodate the inability of the terminal to receive communications while being in a mode of transmitting communications. A frame that is used for the forward link, is divided into N—for example 4—equal length sub-frames. A forward link stream carried by each sub-frame will serve 1/N—one fourth using the same example—of the terminal population in a beam. The satellite return link scheduler will assign capacity to terminals, while taking into account their sub-frame association. This scheme simplifies scheduling by the satellite and allows the terminals to be grouped for addressing over the forward link, and to save receiver power.
A forward link super-frame structure, taken together with signaling e.g. over a DVB-S2 (or any other applicable standard) PL (“Physical Layer”) header, is used to alert terminals which are in stand-by mode to a forward link traffic that is queued and is about to be transmitted to them.
Beam and satellite handover may optionally but not necessarily rely on a system-wide GPS-grade time-base; terminal geo-location information; accurate satellite orbital data, communicated to the terminals through layer 2 signaling over the forward link; and the framing scheme described hereinabove. These enable the gateway and the terminal, running both identical, bit-exact coverage calculation routines, to be synchronized for traffic routing and beam/satellite selection that requires minimal signaling.
Beam or satellite switchover for terminals that are in a stand-by mode or are currently receiving data, will involve no signaling and will be done with no interruption to the traffic. Return link transmission during switchover involves exchanging modified capacity request messages. It is preferably seamless during intra-satellite switchover and nearly so between satellites.
According to an embodiment of the present disclosure there is provided a method for enabling communications between one or more satellites and a plurality of terminals wherein the plurality of terminals are divided into M groups of terminals and wherein the method comprising:                forwarding a plurality of communication frames in a forward link, wherein the plurality of frames are divided into N sub-frames, and wherein traffic being carried along the forward link by each of the N sub-frames serves one or more groups of terminals associated with a respective satellite, and        assigning, by a satellite return link scheduler, a respective capacity of the return link for at least one of the one or more groups of terminals, wherein the assignment takes into account which of the sub-frames is associated with that at least one group of the terminals.        
According to another embodiment, the terminals belonging to the at least one group of terminals are characterized in that they cannot receive communications while they are transmitting communications.
In accordance with another embodiment, each of the at least one group of terminals is further divided into sub-groups, and a Physical Layer Header (PL-Header) of each of the forward link communication frames specifies at least one of the sub-groups, and wherein each communication frame carries traffic addressed to the at least one sub-group specified in the respective PL-Header.
By still another embodiment, each terminal is configured to decode every PL-Header of the forward link communication frames, and wherein the method further comprises a step whereby if the PL-Header carries a an indication of a sub-group that matches the sub-group of terminals to which a respective terminal belongs, the respective terminal will decode the entire communication frame, and if the PL-Header carries an indication of a sub-group that does not match the sub-group of terminals to which a respective terminal belongs, the respective terminal will not decode the respective entire communication frame.
In accordance with yet another embodiment, in a case where the PL-Header carries an indication of a sub-group that does not match the sub-group of terminals to which a respective terminal belongs, the respective terminal is configured to power down its receiver for the duration of the entire communication frame.
According to another embodiment, the method provided further comprises a step of alerting terminals from among the plurality of terminals which are currently in a stand-by mode, that traffic that is destined to them is currently being queued and is about to be transmitted to them.
In accordance with yet another embodiment, each of the N sub-frames comprises a baseband frame, and wherein all of the base-band frames are of a fixed, pre-defined length, having different modulations and/or different codes.
In accordance with another aspect, a method is provided for enabling communications between one or more satellites and a plurality of terminals, wherein the one or more satellites are configured to communicate with the plurality of terminals belonging to a public network through at least one gateway, and wherein the plurality of terminals and the at least one gateway are configured to execute identical, bit-exact satellite coverage calculation routines, synchronized for traffic routing and beam/satellite selection with minimal signaling.
According to another embodiment of this aspect, each of the plurality of terminals is configured to generate requests for allocation of return link capacity in another beam or a different satellite, thereby when a terminal switches a beam or a satellite, it is able to immediately utilize said allocated capacity over the new (switched-to) beam or at the new satellite.
In accordance with another embodiment, the terminal is configured to:                accept initial geolocation information and to carry out a coarse alignment procedure; and        execute a calibration routine that allows fine-aligning of its orientation and tilt based on reception of communications sent by the terminal to the respective satellite.        
By yet another embodiment, the one or more satellites are configured to:                use gateway-referenced mechanism to establish a system-wide Time of Day (ToD) time base; and        to periodically broadcast information that specifies the information that relates to a respective satellite of the one or more satellites.        
According to still another embodiment, adaptive acquisition time is allocated for a period of time required for carrying out an inter-beam switchover and/or inter-satellite switchover.
In accordance with another embodiment, the satellite system is a member selected from a group that consists of: a Geo Stationary system, a LEO system and a MEO system.